This invention relates to the field of power transmission and distribution equipment, more particularly to the field of protective devices for power transmission and distribution equipment, and more particularly still to the field of localized protection of power transmission and distribution equipment, such as transformers and surge arrestors.
Transformers placed on poles, or at remote distribution sites, are subject to the ravages of weather with little to protect them. When storms create high winds, adjacent wires between poles can touch, which creates a surge in the line which can cause a short in the transformer. Likewise, if lightning hits a wire or the transformer, a violent transformer destroying electrical surge will pass through an unprotected transformer. To help prevent these disruptive weather related occurrences from destroying the distribution equipment, surge arrestors and fuses are placed in the distribution network immediately adjacent the transformer, or other component, which is to be protected.
Fuses are used to protect the transformers located on poles or at remote locations in the distribution network. Power lines coming into the transformer are typically rated at between one half and 100 amps, having voltages of between 2400 and 38000 volts. These fuses are commonly placed in a parallel electrical relation to the surge arrestor. The fuse, and transformer, are mounted in series. These two components in series are then mounted in an electrically parallel configuration to the surge arrestor. The arrestor provides protection to the transformer by diverting surges, such as those caused by lightning, to ground, rather than through the fuse and transformer and into the rest of the distribution network. When a large amperage surge occurs, such as that caused by a lightning strike on the distribution power line, the surge arrestor will change its impedance to create a very low resistance path to ground. The surge will thus pass primarily through the surge arrestor, which has a substantially lower impedance and resistance than the fuse and transformer combination. The fuse provides long term overload protection to the circuit, such as what occurs when a short appears as a result of a failure in the transformer or a long term over load situation is present in the secondary circuit. However, the fuse is not intended to carry the lightning surge to ground. The prior art fuses cannot withstand the full surge current created during a lightning strike, and thus the surge arrestor must be placed in parallel with the series combination of the transformer and fuse to protect both the transformer and the fuse.
To physically locate the surge arrestor and fuse-transformer combination in a parallel electrical configuration, the surge arrestor and fuse are both placed upon the pole, or otherwise remotely located from the transformer tank, and the ground lead is run from the transformer tank to the surge arrestor. This arrangement leads to less protection of the transformer secondary windings than would be present if the surge arrester were mounted directly on or in the transformer tank. It is known that the longer the length of the lead between the transformer and surge arrestor, the greater the likelihood of damage occurring to the secondary transformer windings during a current surge condition. However, the prior art fuses dictate that the surge arrestor be remotely mounted, in order that the surge arrestor and fuse may be mounted in parallel to protect the transformer and fuse.
The individual fuse associated with a transformer must be sized to protect the transformer, but not prematurely open in response to rated amperage or low overload conditions. Each transformer will have a specific rated primary amperage and voltage which must pass therethrough to provide the proper total voltage and amperage on the secondary, or low voltage, side thereof. Likewise, as the rated amperage and voltage of the transformer varies from application to application, the fuse which protects the transformer must be sized to match the performance rating of the transformer. Therefore, the fuse manufacturer typically must supply a line of fuses with different opening amperages for transformer protection. These requirements are well known in the art, and handbooks, design manuals and government and industry standards are promulgated which dictate to designers the power absorption, time to blow, and overcurrent characteristics of fuses for high energy applications.